to go up to, flourishing a superlative
郅 centers on reaching a peak or extreme: going up to a high point, or flourishing to a superlative degree. Both senses share the idea of attaining a notable level.
郅 combines 至 (arrive, reach) with ⻏ (阝, a mound or place), suggesting reaching a high place or a flourishing settlement. The exact historical development is uncertain.
The left side 至 means 'arrive' or 'reach', and the right side ⻏ looks like a mound or hill. Picture climbing to the top of a hill and flourishing there, reaching a superlative state.
For シツ, imagine reaching the summit and shouting 'Sheets!' as you spread out a picnic blanket to celebrate your flourishing achievement: sheets -> シツ.